CMOS image sensors based on an active pixel design have gained wide acceptance in camera applications. In such sensors each pixel in the final image is generated by a pixel sensor that includes a photoreceptor that accumulates charge during an exposure. The accumulated charge is converted to a voltage by an output amplifier that is typically constructed from a source follower transistor that receives the charge at its gate and drives a bit line that is connected to the readout circuitry in the imaging array. The signal on the bit line is then digitized using an analog-to-digital converter (ADC) that is connected to the bit line.
In cameras that utilize global shutters, the image is projected on an array of pixel sensors after all of the photodiodes in the pixel sensors have been reset. All of the photodiodes accumulate charge in an amount that depends on the light intensity received by that photodiode. At the end of the exposure, the charge accumulated by each photodiode is transferred to a floating diffusion node in the pixel sensor, and the photodiode is isolated from the floating diffusion node, thereby ending the exposure. The charge remains on the floating diffusion node until the pixel sensor in question is readout. During the storage of charge on the floating diffusion node, the node accumulates additional electrons from noise sources in the array of pixels and the surrounding processing circuitry. The contribution of such “noise electrons” to the final image sensor intensity value depends on the amount of time the charge remains on the floating diffusion node. Compensation for this noise source remains a challenge in the search for designs that reduce the overall noise in CMOS imagers.